1. Field of the Invention
This invention relates to the remedial treatment of coal bed methane wells to enhance the production of methane therefrom.
2. Description of the Prior Art
The production of methane from subterranean coal beds by way of wells drilled into the coal beds has become more common in recent years. Such coal beds contain adsorbed hydrocarbon gases consisting primarily of methane (about 95%) with the remainder being ethane, propane, higher homologs and sometimes CO.sub.2 and N.sub.2. The coal beds are reservoirs for a great amount of gas which can be produced by way of well bores which are drilled and completed using conventional techniques. The coal beds usually contain water which when produced causes pressure to be reduced and the methane and other gases to be desorbed from the coal and produced along with the water.
While different stimulation techniques have been utilized by the industry to enhance production of methane from coal bed wells, conventional hydraulic fracturing techniques are the most common. In hydraulic fracturing, a fracturing fluid such as an aqueous gel or foam is pumped through a well bore and into a producing zone in a coal seam penetrated by the well. The fracturing fluid is pumped at a rate and resulting pressure sufficient to create fractures in the coal seam. The fractures are extended by continued pumping, and typically, a proppant material such as sand is mixed with the fracturing fluid and carried into the fractures thereby. When the pumping is stopped and the pressure on the producing zone is released, the induced fractures are propped open by the proppant material thereby providing flow channels through which produced fluids flow to the well bore.
As mentioned, the produced coal bed fluids generally include water in addition to the desorbed methane and other gases. The water contains dissolved minerals, and when the water is subjected to pressure and temperature changes as it flows through the coal formation and through perforations communicating the formation with the well bore, the minerals may precipitate and produce scale deposits within the formation and in the perforations. Such scale deposits often partially or totally plug formation flow channels as well as the perforations whereby the flow of produced fluids is severally restricted or stopped. In addition to the scale deposits which precipitate from the produced water, damage to the coal bed producing zones often occurs as a result of drilling the well bore and the performance of completion and stimulation procedures. For example, debris such as filter cake from the drilling fluid utilized during the drilling of the well bore and unbroken gels utilized in completion and stimulation techniques are often left in the coal formation. Such debris can partially plug the near well portions of producing zones and reduce production.
Most coal bed methane wells penetrate a plurality of coal seams which include one or more producing zones communicated with the well by perforations therein. The production of methane from all of the producing zones is generally necessary for a particular well to be economically attractive. However, after a coal bed methane well has been produced for an initial period of time, the above mentioned drilling, completion and stimulation debris left in the coal formations and the mineral scale deposits as well as coal fines which migrate with the produced fluids often combine to plug the perforations and producing zones. Heretofore, in order to perform remedial treatments to unplug the perforations and producing zones, it has been necessary to isolate the producing zones using packers and the like which involves considerable time and expense. Also, the remedial treatments carried out have often been ineffective in removing the scale deposits and other plugging solid materials from the producing zones and perforations.
Thus, there is a need for improved remedial treatment methods for coal bed methane wells which effectively enhance production and are relatively inexpensive to carry out.